This invention relates to apparatus for accurately controlling a number of current sources and, more particularly, to a technique by which each current source is periodically addressed by an associated control signal by the use of a time sharing arrangement.
Background Description:
Analog circuits such as variable gain amplifiers, variable attenuators, and active equalizers are often controlled with variolossers, current controlled amplifiers, programmable operational amplifiers, and other similar devices. However, the operation of these devices requires variable current sources to provide an output current that varies in accordance with some status signal.
The application of such control devices is often found in telecommunication systems where pilot signals are used for monitoring the transmission characteristics of all the circuits in the signal path of interest. Pilot detectors monitoring such pilots provide DC output signals, the amplitude of which is proportional to the level of the pilots selected. These DC signals are used to control or drive multiple current sources which in turn control the transmission function of the analog circuits mentioned above (variable gain amplifiers, etc).
In actual operation, a change in ambient temperature might cause, for example, an unwanted change in system gain. Variations in pilot levels caused by a system gain change would be detected by pilot detectors. Change in DC output signals from pilot detectors in turn would produce corresponding changes in gain or loss of amplitude sensitive devices (amplifiers or attenuators).
This technique is adequate for circuits where some degree of error (caused by mismatched components and temperature variations) is tolerable. In a system where only very small changes in level can be tolerated, this technique is inadequate, particularly where a system level alignment cannot be made. It is an object of this invention to provide apparatus for controlling a system parameter, such as system gain, where only very slight changes in the system parameter can be tolerated.